Manganese activated zinc silicate (Zn.sub.2 SiO.sub.4 :Mn.sup.2+) is an efficient green emitting photo- and cathodo-luminescent phosphor. In mineral form, zinc silicate is known as Willemite. When doped with Mn, it is commercially designated as P1 (short persistence phosphor for cathode ray tubes (CRTs) and lamps). When doped with Mn and As, Willemite is commercially designated as P39 (long persistence phosphor for special CRTs). This phosphor has been studied extensively and is currently used in plasma display panels (PDPs), CRTs and lamps due to its high quantum efficiency, persistence characteristics, color purity and reduced saturation. Mn activated Willemete has a rhombohedral structure (space group R3). The Zn.sup.2+ ions occupy two in-equivalent sites, both having four oxygens (nearest neighbor) in a slightly distorted tetrahedral (Td) configuration. The emission (green) is attributed to the reversal of a d-orbital electron of the Mn.sup.2+ (substitution of Zn.sup.2+) ion. See D. T. Plumb and J. J. Brown, J. Electrochem Soc. 117 (1970) 1184.
These phosphors are conventionally prepared by high temperature (&gt;1200.degree. C.) solid state reaction (SSR) between ZnO (Zn source), SiO.sub.2 (Si source), Mn.sub.2 O.sub.3 or Mn(NO.sub.3).sub.2 (Mn source) and NH.sub.4 F/NH.sub.4 Cl (flux). The grain size of the phosphor powders prepared by SSR are on the order of 5 to 20 microns. Flat panel display devices such as PDPs, field emission displays (FEDs) and electro-luminescence (EL) panels require thin fluorescent screens with fine grain (0.1 to 2 microns) phosphors for better performance and high efficiency. This requirement is more demanding in the case of PDPs, as the phosphors are screen printed between complicated structures, such as ribs. With small particles, it is possible to form a thin screen. Small particles also allow for a higher packing density and less binder content.
Originally, phosphors having a small particle size were obtained by grinding, crushing or milling large phosphor particles. Phosphors obtained by these methods displayed greatly reduced efficiency, with little or no control over the particle morphology. More recently, "no mill" phosphors have been prepared by rapid cooling of a phosphor mass after completion of SSR and with either a short-time firing at a higher temperature or a longer duration firing at a lower temperature. These processes help in minimizing further growth of phosphor crystals. In the presence of flux or inhibitors, particle size distribution (PSD) and morphology of the phosphor can be controlled. See M. Kotaisamy, R. Jagannthan, R. P. Rao, M. Avudaithai, L. K. Srinivasan and V. S. Sundaram, J. Electrochem Soc. 142 (1995) 3205; R. P. Rao, J. Electrochem Soc. 143 (1996) 189. It has been proposed that sub-micron particles can be synthesized by a sol-gel process. See T. R. N. Kutty, R Jagannthan, R. P. Rao, Mater. Res. Bull. 25 (1990)1355. Small phosphor particles have been synthesized by hydrothermal methods. See R. N. Bhargava, D. Gallagher, T. Welker, J. Luminescene 60 (1994) 280.
Most past work on zinc silicate phosphors has been related to fluorescent lamp development and the performance of the phosphor therein, either alone in green lamps, or in white lamps with phosphor blends having zinc silicate as one component. Different methods of preparation and the introduction of various impurities were tried in attempts to improve the life of the lamp. U.S. Pat. No. 4,208,448 to Panaccione teaches that the life of the phosphor is improved by washing the phosphor with an organic acid solution (e.g., acetic, succinic or terephthalic acid) before application to the inside of the lamp envelope. Trace amounts of alkali metals, such as Mg, have been added to zinc silicate in addition to Mn and As to obtain superior persistence at higher drive levels. A practical application of this phosphor is in CRTs (see U.S. Pat. No. 4,315,190).
U.S. Pat. No. 4,440,831 to Brownlow et. al. describes an improved process for synthesizing zinc silicate in which phosphor particles are formed by using small size silicic acid particles coated on ZnO and Mn.sub.2 O.sub.3 along with H.sub.2 O, H.sub.2 O.sub.2, HNO.sub.3 and NH.sub.4 OH and fired at high temperatures. This phosphor was shown to display increased brightness. However, persistence also increased compared to conventional phosphors. The addition of alkaline earth elements (Mg, Ca, Sr), Na and either Bi or Sb, along with Mn and As, to a zinc silicate phosphor minimizes degradation and allows for easy blending with other phosphors (blue and red) in a special type CRTs according to U.S. Pat. No. 4,551,397 to Yaguchi et. al. A small quantity of tungsten has been shown to improve lamp life and brightness (U.S. Pat. No. 4,728,459). The application of a non-particulate, conformal aluminum oxide coating to the outer surface of individual particles also improves lamp life (see U.S. Pat. Nos. 4,892,757, 4,925,703 and 4,956,202 to Kasenga et al.). The reflectance of phosphor particles before surface treatment can enhanced by washing the phosphor particles with citric acid (U.S. Pat. No. 5,039,449). U.S. Pat No. 5,188,763 to Chenot discloses that the addition of MgF.sub.2 to a NH.sub.4 Cl flux in the starting ingredients, is essential for achieving a white bodied phosphor. U.S. Pat. No. 5,611,961 to Forster et al. describes the synthesis of a zinc orthosilicate using fumed silica having an ultra fine average particle size of less than 50 nm by firing at higher temperatures in an inert atmosphere.
As noted above, most of the earlier patent work on zinc silicate phosphors relates to lamps and long persistence (&gt;20 ms) CRTs. However, long persistence phosphors are not suitable for today's high performance TV display applications, as long persistence creates superimposed images (a ghost effect). Therefore, there is a need for a phosphor capable of achieving a 5-10 ms persistence (the sensitivity of human eye for video images) without sacrificing brightness. The optimum concentration of Mn in the phosphor is very critical. After exceeding a specific Mn concentration, there is a marked decrease in brightness, although persistence will decrease continuously with increased Mn concentration. This forces a tradeoff between brightness and persistence. Also, since most synthesis methods involve high temperature solid state reactions, the control over the impurity concentration (effective doping of Mn.sup.2+ into the crystal lattice), PSD and morphology is limited. It has been found that phosphor screens formed with smaller particles (0.5 to 2.0 microns) exhibit improved performance. This is particularly true for PDPs. However, most of the above methods fail to provide small particles (0.1 to 2.0 microns).